MEMS devices are becoming more widely used in many applications. Some of the applications require some of the MEMS components to move with relatively large amplitude in a lateral direction, or in-plan direction, sometimes also referred as horizontal direction. Here the definition of lateral direction, in-plan direction, or horizontal direction, is generally in reference to MEMS manufacturing processes. In typical MEMS manufacturing, various thin film layers (metal and dielectric) are deposited onto a starting substrate. Patterning and etching processes are applied to the thin film layers to form venous patterns in each layer. The patterns and the layers are device specific. The layer-by-layer stacking direction, perpendicular to the surface of starting substrate onto which the thin film layers are deposited, is generally called vertical direction. The directions within the layers, parallel to the surface of the starting substrate (perpendicular to film stacking direction), are called in-plan, lateral, or horizontal directions. Subsequent discussions follow this convention. The term of in-plan direction, lateral direction and horizontal direction may be used interchangeably.
For example, in display industry, several methods are being developed to use MEMS devices for display application to improve energy efficiency over liquid crystal display (LCD) and plasma display (PD). This is motivated by the fact that both LCD and PD are relatively inefficient in energy use. For example, polarizer alone in LCD reduces light intensity by fifty percent, while PD consumes significantly more power per lumen than LCD. There exist a number of disclosures and publications on using MEMS devices for display applications with substantially reduced energy consumption. One example was recently disclosed in a US patent (U.S. Pat. No. 7,995,263 B1) in which a method of using light separators and MEMS light shutters for image display was disclosed. The light separator is used to direct light illuminated on its incident surface into individual pixels, and to condense the light within each pixel to a small light exit groove, which takes only a small fraction of pixel area on the exit surface (viewing surface) of the light separator. Light shutters are built on top of the light distributor to control the color and intensity of light at individual pixels. In this cited example, torsional light shutters swinging around their hinges are employed as examples to control the color and intensity of light. It would be more efficient and advantageous to use shutters with relatively large amplitude of in-plan motion (horizontal motion). For example, an amplitude of horizontal motion greater than 10 um for pixel size of around 80 um, and larger amplitude of horizontal motion for larger pixel sizes.
In order for shutters to have horizontal motion, it is generally necessary to have a force in horizontal direction to move them between their “ON” and “OFF” positions. If the force is of electrostatic nature, this would generally require either a large interaction surface oriented perpendicular to the direction of the horizontal motion or a large driving voltage, or their combination of, in order to generate a force large enough to move the shutters between their “ON” and “OFF” positions. This is because electrostatic force is proportional to the area of the interacting surfaces, but inversely proportional to the square of their separation. A large separation between the interacting surfaces (associated with the large amplitude of horizontal motion of one of the interacting MEMS components) requires larger interaction area and/or higher voltage in order to generate a force large enough to move the shutters.
Large interaction surface oriented in the direction perpendicular to horizontal motion requires a structure that is essentially similar to a vertical wall. This structure can be made available but generally at the cost of increased manufacturing complexity; while applying high voltages across interacting surfaces will generally consumes more electric power and may also cause device reliability issues. In some applications such as mobile application of handheld devices, applying high voltage to any part of the device is simply not practical.
On the other hand, large electrostatic force in vertical direction can be obtained more easily in MEMS devices. A typical MEMS manufacturing process flow involves layer-by-layer semiconductor processes. Large interaction surface in a thin film layer (i.e., the normal of the surface is in the same direction as film stacking direction) can easily be made available. The separation (distance) between different layers can be controlled with an accuracy of well below a fraction of micron. Thus the separation between interacting surfaces can be made small and precise for two adjacent layers interact with each other. As a result, large interaction force in vertical direction can be obtained with small separation between the interacting surfaces and large interacting surfaces area.
A vertical force will generally only result a relative displacement of the interacting surfaces in vertical direction. If the separation between interacting surfaces is small, the amplitude of vertical displacement is also small. In many applications, it requires some MEMS component to move in horizontal direction with large amplitude. It would therefore be useful to have a method that can effectively convert a small vertical displacement in some part of a MEMS device into motion of other MEMS components in horizontal direction and with larger amplitude.